Expand the iframe focusable rule to include scrollable non-interactive element in an iframe

[shunguoy]

The current iframe focusable rule is limited to navigating interactive elements for an iframe with a tabindex='-1'.
The current scrollable rule applies to scrollable elements that do not cover an iframe element.

A gap exists where an iframe contains scrollable content with a tabindex='-1'. In this case, a keyboard user is unable to view the full content through the scroll bar, regardless of whether the content contains any interactive element or not.

The recent Chrome updates add auto-focusable to iframe and scrollable element. This makes the major browsers consistent in the keyboard navigation of iframe and scrollable elements. However, when an iframe has a scrollable bar or contains scrollable content, the iframe is not automatically focusable in either Chrome or Firefox. This looks like a browser bug.

[carlosapaduarte]

CG decision: create a rule for scrollable iframes to address iframes that need to be scrolled which is different from iframes with content that needs to be reached